Ultrasonic cleaning

ABSTRACT

Ultrasonic cleaning of a surface is carried out by applying a film of liquid to the surface and directing ultrasonic energy into the film to cause cavitation thereof and subsequently rinsing the surface. Apparatus for cleaning small articles includes nozzles for spraying water over the articles, a compressed air transducer for generating the ultrasonic waves, a spray rinse for removing loosened surface dirt and a conveyor for moving the articles sequentially past the initial spray, the transducer and the spray rinse.

United States Patent [191 Powell, Jr. et al.

[451 Nov. 19, 1974 ULTRASONIC CLEANING [76] Inventors: Harry C. Powell,Jr., Faber, Va.

22938; Donald W. Floyd, Rt. 2, Box 36, Afton, Va. 22920 221 Filed: Aug.3, 1972 21 App1.No.:277,640

[52] US. Cl 134/1, 15/313, 15/3.2, 99/451, 134/6, 426/237, 426/238,426/286 [51] Int. Cl A23n 13/00, B08b 7/02 [58] Field of Search 134/1,184, 199; l5/3.1, 15/316, 3.17, 3.2, 3.21; 99/451, 1, 217,100

R; 198/22, 22 R, 229; 259/DIG. 44; 426/235,

[56] References Cited UNITED STATES PATENTS 2,213,489 5/1938 Durand15/3.2 2,477,006 6/1943 Pierson 2,806,246 9/1957 Simjian 2,881,0804/1959 Simjian 3,102,290 9/1963 Sannes 3,440,094 4/1969 Adam et al.134/1 OTHER PUBLICATIONS B. Brown and J. Goodman, High IntensityUltrasonics Industrial Applications, ILIFF Books Ltd., London, 1965, pp.5960.

Primary Examiner-Morris O. Wolk Assistant Examiner-Barry I. HollanderAttorney, Agent, or FirmCushman, Darby & Cushman 57] ABSTRACT Ultrasoniccleaning of a surface is carried out by applying a film of liquid to thesurface and directing ultrasonic energy into the film to causecavitation thereof and subsequently rinsing the surface. Apparatus forcleaning small articles includes nozzles for spraying water over thearticles, a compressed air transducer for generating the ultrasonicwaves, a spray rinse for removing loosened surface dirt and a conveyorfor moving the articles sequentially past the initial spray, thetransducer and the spray rinse.

4 Claims, 4 Drawing Figures ('am zezss'eu was I "/51 tit/t ULTRASONICCLEANING This invention relates to wet-cleaning of surfaces and inparticular to cleaning processes and apparatus in which the cleaningaction of a liquid is enhanced by the use of ultrasonic energy. The useof ultrasonic energy in wet-cleaning operations is known. One commonsystem employs an immersion tank in which the object or objects to becleaned are immersed in a liquid following which the mass of liquid inthe tank is exposed to sonic or ultrasonic vibrations. The vibrationsare usually generated by a transducer, suspended in the liquid, whichconverts electrical energy into mechanical vibrations in theneighborhood of 20 kilocycles/second. The vibrations impart kineticenergy to the liquid with the result that there is a rapid formation andcollapse of millions of very small bubbles, referred to in the art ascavitation. The collapse of the bubbles on the surface of an objectimmersed in the body of liquid loosens and removes surface accumulationsof dirt and other material.

It is also known in the ultrasonic cleaning art to supply ultrasonicvibrations to a pressurized stream of liquid admixed with a gas which isthen impinged on the surface to be cleaned.

In the above-summarized cleaning techniques, large amounts of liquid,either the entire tank full or the entire stream, are supplied withultrasonic vibrations, even though only a portion of the cavitatedliquid actually contacts the surface to be cleaned. The result is that asignificant portion of the power input to the liquid is not utilized forcleaning. The present invention overcomes this disadvantage by providinga filmcavitation technique in which only a film of liquid on the surfaceis supplied with ultrasonic energy. More specifically the techniquecomprises wetting the surface with a film of liquid and directing a beamof ultrasonic vibrations from a transducer through the surroundingatmosphere into the liquid film so as to cause cavitation of the latter.When small articles are being cleaned, the articles are caused to rotatewhile in the beam so that all sides of the articles will be exposed to acavitating film. In order to concentrate the cavitating energy in thelocation where needed the transducers will usually be provided with afocusing means.

The technique has particular utility in the cleaning of soft articlessuch as fruit and certain other food products for several reasons. Ingeneral, soft materials tend to absorb ultrasonic energy, and when suchmaterials are placed in an immersion tank type of ultrasonic cleaningsystem they may absorb so much of the power that not enough is left forproducing cavitation of the liquid. A given system can generally be madeoperational by increasing the power input to the transducer, but theresulting power per unit volume of liquid is then substantially higherthan is required for the cleaning of hard objects, such as glassware orwatch parts. The film-cavitation technique of the present invention isnot nearly as susceptible to these disadvantages, owing to therelatively small volume of liquid in the film.

Another advantage of the film-cavitation technique with respect tocleaning soft materials is that it permits the use of relatively lowfrequencies (e.g., 3 to I kilocycles/second) which are less costly togenerate than the higher frequencies (e.g., above 100 kilocycles/second.The absorption of ultrasonic energy by soft materials decreases as thefrequency of vibration is increased, but increasing the frequency is notalways economical because the amount of power required for initialcavitation is higher for the higher frequencies.

This consideration is not of great significance in the film-cavitationtechnique because of the small volume of liquid in the film.

In one preferred embodiment of the present invention a compressed airtransducer is employed to produce ultrasonic vibrations in the range ofabout 3 to about 30 kilocycles/second in air. Higher frequencies may beemployed if the atmosphere through which the vibrations are transmittedis, for example, helium. This type of mechanical transducer is capableof efficiently propagating ultrasonic vibrations through gases and ofproducing a beam which can be focused rather easily. Electricaltransducers of the electro-mechanical, magnetorestrictive andpiezo-electric type may also be used, although for many applicationsthese devices are inefficient.

The invention will be further understood from the following moredetailed description of an illustrative embodiment taken with thedrawing in which:

FIG. 1 is a schematic longitudinal sectional view of an apple-cleaningdevice embodying the principles of the present invention;

FIG. 2 is a schematic transverse sectional view of the apparatus of FIG.1;

FIG. 3 is a fragmentary view of the transducer of FIG. 1; and

FIG. 4 is an end view of the transducer.

For purposes of illustration the invention is hereinafter described interms of an apparatus for cleaning apples but it will be understood thatthe principles of the film-cavitation technique are applicable to thecleaning of any surface, including the surface of other soft objects andof hard objects and thesurface of large stationary surfaces either flator curved.

In FIG. 1 there is illustrated an apple-cleaning ma chine which includesa tank 10 and conveyor and brushing means in the form of two parallelrotating brushes 12 and 14 disposed above the bottom of the tank 10 andextending longitudinally of the tank 10. The brush-type conveyor may beof a conventional construction of the type which acts on objects, forexample apples 16, placed in their nip to move the objects along thelength of the brushes in the direction of the arrows whilesimultaneously and continuously brushing the surface of the objects andcausing the objects to rotate so that all portions of the surfaces arebrushed. To this end one of the brushes 12 or 14 should have itsbristles arranged in a spiral pattern, and both brushes l4 and 12 shouldbe driven, as indicated schematically by a motor M. Conveniently theshafts of the brushes may be joumaled in bearings 18 secured to the endwalls of the tank 10.

Above the brushes l4 and 12 is disposed a liquid spray system forapplying a film of wash liquid to the apples l6 and for subsequentlyrinsing the apples 16. The system may take any convenient form such as apair of horizontal manifold pipes 20 on which nozzles 22 and 24 areprovided. The nozzles 22 are located above the upstream ends of thebrushes l2 and 14, and the rinsing nozzles 24 are located above thedownstream ends of the brushes l2 and 14. Wash liquid is supplied to themanifold pipes 20 in any convenient manner either from a source (notshown) or from the lower portion of the tank by means of a pump 26.

lntermediate the nozzles 22 and 24, and preferably closer to theupstream nozzles 22 is a transducer 28 for generating ultrasonicvibrations and beaming them through the surrounding air to the film ofwash liquid on the apples 16 passing beneath the transducer 28. In thisillustrated embodiment the transducer 28 is a mechanical transducer ofthe I-Iartmann type, operated from a source of compressed air andutilizing the same principle as a steam or air whistle or organ pipe.This type of transducer offers the advantages of simplicity, ease ofmanufacture, high power output that is easily focused and simplycompressed air for the power supply. While it may suffer from a lack ofpositive frequency control, this is not critical in most cleaningsystems.

As shown in FIG. 3 the transducer 28 includes two main parts, an air jetmember 30 and a resonator 32. The jet member 30 is constructed of ahollow cylinder having a conical end portion 34 which terminates in asmall orifice 36, and a cylindrical end connected to a source ofcompressed air. The cylindrical end is fixed within an outer sleeve 37,as by means of set screws (not shown). The bore of the jet 30 taperstoward the orifice 36 as seen at 38.

The resonator 32 is a solid conical member arranged coaxially with thejet member 30 and provided at its apex end with a small cylindricalcavity 40 which faces the orifice 36. The opposite end of the resonator32 terminates in a circular flange 42. The jet member 30 and theresonator 32 are fixed in position by means of three tie rods 44 havingthreaded ends which extend through unthreaded holes in the flange 42 andin the sleeve 37. Nuts 46 and 48 engage opposite surfaces of the flange42 and sleeve 37, respectively, to fix the latter members in place andto allow for adjustment of their longitudinal positions during tuning ofthe system for greatest power output.

This type of transducer generates vibrations in the pattern of a thincircular disc, with its center at the orifice 36. In order toconcentrate the energy in the form of a beam 49 at the surface of theapples 16 passing beneath the transducer, a reflector in the fomi of aparabolic metal strip 50 is disposed around the orifice 36. The strip 50is set at a small angle to the plane of the orifice 36, as seen in FIG.3, because the disc-shaped energy pattern radiating from the orifice 36is in a plane which lies at an angle of 1015 to the plane of the orifice36. The strip is fixed in position in any suitable manner, as by havingits rear convex surface welded or brazed to two of the tie rods 44.

In operation of the apparatus the brushes l2 and 14 are rotatably drivenby the motor M, and the apples 16 are fed sequentially to the uppersurfaces of the brushes at the inlet end of the tank 10. An inlet chute52 may be provided for feeding the apples 16 to the nip of the brushes.Rotation of the brushes revolves each apple 16 rapidly and at the sametime moves it to the left toward the outlet end of the tank 10 where anoutlet chute 54 may be provided. Prior to and after passing beneath thetransducer 28 each apple 16 is sprayed with wash liquicl, for examplewater, which drains downwardly through the brushes into the tank 10. Theliquid applied by the nozzles 22 drains away before the respectiverevolving apple 16 enters the energy beam 49 from the transducer so thatthe beam 49 acts only on the film of liquid remaining on the apple 16.The resulting cavitation of the film, which requires relatively lowpower owing to the small liquid volume of the film. loosens surfaceaccumulations of dirt and natural waxes on the apple 16, and theseaccumulations are subsequently flushed into the tank 10 by liquid beingsprayed from the nozzles 24.

While not illustrated in the drawing the cleaning technique may includeseveral nozzles 24 and transducers 28, or more than one pass of theapples l6 beneath the transducer 28, so as to expose the apples 16 to asequence of cavitation and rinsing operations.

Referring to the operation of the transducer 28 it has already beenexplained broadly that the latter produces a disc-shaped beam 49 ofultrasonic vibrations in the surrounding air which is focused downwardlyonto the apples 16. Specifically, the air escapes from the orifice 36 atsupersonic velocity and sets up regions of instability in the airstream. The effect of the resonator cavity 40, which is disposed in theair stream at a region of instability is to excite the air stream intooscillation at a frequency that is dependent upon the dimensions of theresonator cavity 40. In the illustrated embodiment supersonic airvelocity through an orifice 36 of 0.1162 inches in diameter was obtainedwith air pressure of about 300 psi, using about 5 cubic feet of air perminute. The resonator cavity 40 was of the same diameter, and underthese operating conditions the device generated about 55 acoustic wattsat about 20 kilocycles per second. The dimensions of the apparatus areof course not critical, and as explained previously, frequencies ofbetween 3 and 30 kilocycles per second in air are suitable.

The invention has been illustrated in simplified form and it will beunderstood that various modifications may be made to the illustratedprocess and apparatus without departing from the scope of the invention.

What is claimed is:

1. Apparatus for ultrasonically cleaning the surfaces of articlescomprising: a transducer for generating a beam of ultrasonic vibrationsin the surrounding atmosphere; conveyor means for transporting anarticle transversely through the beam and for simultaneously revolvingand brushing the article, said means including two closely spacedparallel brushes rotatably driven in the same direction; liquidapplication means for wetting the surface of the article carried by theconveyor means with a film of liquid before the article enters the beam;and rinse means for rinsing the article after it has been exposed to thebeam.

2. Apparatus as in claim 1 wherein said transducer is a mechanicaldevice which generates the beam of vibrations from a stream ofcompressed gas.

3. Apparatus as in claim 1 wherein said liquid application means andsaid rinse means include nozzles for spraying liquid on to the revolvingarticle as it is being conveyed by said brushes.

4. A method for removing surface dirt from articles of fruit comprising:wetting the surfaces of the fruit with a film of liquid; conveying thewetted fruit along a path while simultaneously revolving and brushingall surfaces of the fruit; generating a beam of ultrasonic vibrationsand directing the beam through the surrounding atmosphere into the filmof liquid while the articles of fruit are being conveyed, revolved andbrushed so as to cause cavitation of the liquid film and thereby loosenmaterial on the surface of the articles by the cavitation and thebrushing; and rinsing the surfaces of the articles with liquid to removeloosened materials while the articles are being conveyed, revolved andbrushed.

1. APPARATUS FOR ULTRASONICALLY CLEANING THE SURFACES OF ARTICLESCOMPRISING: A TRANSDUCER FOR GENERATING A BEAM OF ULTRASONIC VIBRATIONSIN THE SURROUNDING ATMOSPHERE; CONVEYOR MEANS FOR TRANSPORTING ANARTICLE TRANSVERSELY THROUGH THE BEAM AND FOR SIMULTANEOUSLY REVOLVINGAND BRUSHING THE ARTICLE, SAID MEANS INCLUDING TWO CLOSELY SPACEDPARALLEL BRUSHES ROTATABLY DRIVEN IN THE SAME DIRECTION; LIQUIDAPPLICATION MEANS FOR WETTING THE SURFACE OF THE ARTICLE CARRIED BY THECONVEYOR MEANS WITH A FILM OF LIQUID BEFORE THE ARTICLE ENTERS 2.Apparatus as in claim 1 wherein said transducer is a mechanical devicewhich generates the beam of vibrations from a stream of compressed gas.3. Apparatus as in claim 1 wherein said liquid application means andsaid rinse means include nozzles for spraying liquid on to the revolvingarticle as it is being conveyed by said brushes.
 4. A method forremoving surface dirt from articles of fruit comprising: wetting thesurfaces of the fruit with a film of liquid; conveying the wetted fruitalong a path while simultaneously revolving and brushing all surfaces ofthe fruit; generating a beam of ultrasonic vibrations and directing thebeam through the surrounding atmosphere into the film of liquid whilethe articles of fruit are being conveyed, revolved and brushed so as tocause cavitation of the liquid film and thereby loosen material on thesurface of the articles by the cavitation and the brushing; and rinsingthe surfaces of the articles with liquid to remove loosened materialswhile the articles are being conveyed, revolved and brushed.